Optical fibers often have a circular section while waveguides often have a rectangular section. One of the main reasons for this is that optical fibers are manufactured by melting a preform, and the circular geometry is most adapted for this manufacturing process. Manufacturing optical fibers by melting a preform is suitable for making long fibers, especially for telecommunications.
However, there are more and more cases where fibers are not sought for the long distance transport (“extra-muros”), but rather for short distances (“intra-muros”, up to a few meters). In that case, the circular section is not mandatory. However, very special properties are required from such fibers. These properties include but are not limited to:
a. transport of very highly energetic laser sources
b. lasing action within the fiber
c. coupling with several sources
d. light transport in the infra-red
e. polarization maintaining/Bragg wavelength selection/DFB lasing
In order to obtain such properties, fiber designs have been proposed in the prior art. One proposal is to replace the traditional index guiding with guiding through engineered reflections (“Theory of Bragg fiber”, P. Yeh, A. Yariv and E. Marom, JOSA 68, 9, 1978). This engineering may be optimized so that complete reflection at every angle is possible. In such a case, it is possible to fabricate an air-core fiber, allowing high energies to propagate in the fiber (“Wavelength-scalable hollow optical fibres with large photonic bandgaps for CO2 laser transmission”, B. Temelkuran, S. D. Hart, G. Benoit, J. D. Joannopoulos and Y. Fink, Nature, vol 420, December 2002, page 650). In order to do so, a possibility is to cover the cladding wall with an omnidirectional mirror. In “Omnidirectional multilayer device for optical waveguiding”, Fink et al., PCT patent application WO 00/22466, such a multilayer is realized using a polystyrene-chalcogenide multilayer coating.
In “High index-contrast fiber waveguides and applications”, Anderson et al, PCT patent application WO 02/084362, such a coating is obtained by using very specific material compositions at very specific conditions in order to match both the optical properties and the mechanical properties between the two materials of the multilayer, which is needed in order to extrude the fiber.
In “Dielectric waveguide with transverse index variation that support a group velocity mode at a non-zero longitudinal wavevector” PCT patent application WO 02/43180, a photonic crystal fiber laser structure is proposed, based on zero-velocity modes. However, it does not show how to efficiently couple light in the photonic crystal fiber.
Another possibility is to create an array of holes that constitutes either a full photonic crystal or at least a strong guiding through a lower refractive index (“A photonic crystal fiber and a method for its production”, Russel et al, PCT patent application WO 00/60388). Such structures are also well suited for lasing action. However, like in the preceding case, coupling the pump to the fiber may be done only through the fiber end (end-pumping) or the side (side pumping), as in “Photonic crystal fibre guiding a first mode and a pump beam”, Russel et al, PCT patent application WO 01/42829.
All the methods currently used are based on drawing fibers, which places stringent conditions on the materials that are used. Moreover, these drawing techniques lead to fibers that do not allow simple optical pumping if several pumping beams must be used.